THE MOEPHOLOGY OF THE CEEEBEAL NERVES. 797 



and arch. The lingual branches -are regarded as the main stem (post-trematic), the pharyngeal 

 branches as subordinate branches ; the tympanic branch being the> prse-branchial or prse-trematic 

 branch for the anterior margin of the third gill-cleft. 



The vagus nerve is generally regarded as representing the fusion of all the branchial nerves 

 behind the glossopharyngeal. Its efferent fibres are in series with those of the glossopharyngeal 

 above and the accessory nerve below, and belong to the lateral series of His. Its afferent fibres, 

 like those of the glossopharyngeal, represent two elements. The ganglion nodosum has possible 

 connexions with epibranchial sense-organs the rest of the nerve representing the fused branchial 

 nerves of fishes. The superior laryngeal nerve is looked upon as the branchial nerve of the 

 fourth, and the recurrent nerve as the branchial nerve of the fifth arch. While the relation 

 of the nerve to the hinder gill-arches and clefts makes it possible to understand the innervation 

 by the vagus of the heart and lungs, no satisfactory explanation is forthcoming of the 



L, and its distribution to the stomach and other organs be 



of the nerve into the abdomen, and its distribution to the stomach and other organs below the 

 diaphragm. 



666. SCHEME TO ILLUSTRATE THE DISPOSITION OF THE MYOTOMES IN THE EMBRYO IN RELATION 

 TO THE HEAD, TRUNK, AND LIMBS. 



I A, B, C, First three cephalic myotomes ; N, 1, 2, 3, 4, Last persisting cephalic myotomes ; C, T, L, S, 

 Co, The myotomes of the cervical, thoracic, lumbar, sacral, and caudal regions ; I., II., III., IV., V., 

 VI., VII., VIII., IX., X., XL, XII., refer to the cerebral nerves, and the structures with which they 

 may be embryologically associated. 



I 



The accessory nerve consists of two parts. The internal ramus (accessory portion) of the nerve 

 , consists of efferent fibres for the branchial region, in series with the lateral motor roots of the 

 glossopharyngeal and vagus nerves. The external ramus (spinal portion) of the nerve is also 

 composed of efferent fibres, and represents the only lateral motor elements arising from the 

 spinal medulla. 



Olfactory Nerve. There is complete uncertainty regarding the morphology of this nerve. 

 !t consists of three elements : (1) the olfactory bulb, derived from the cerebral hemisphere, 

 solid in man, but a hollow cerebral diverticulum in certain animals, and forming the 

 rhinencephalon ; (2) the olfactory ganglion, with its central and peripheral processes, derived 

 from the ectoderm ; (3) the nasal pit. Attention has been specially fixed on the olfactory 

 ; ganglion, which has been compared to (1) a spinal ganglion, derived from the anterior end of the 

 medullary groove ; and to (2) a lateral line sense-organ. 



The optic nerve also presents an insoluble problem in regard to its morphological position 



